Magnetic hard disc substrate and process for manufacturing the same

ABSTRACT

A magnetic hard disc substrate made of an aluminum alloy for memory devices such as a computer must correspond to an improved structure in which the interval between a head and a medium is made narrower to attain high density recording. In order to prevent head crash, it is required that the magnetic hard disc substrate have a smooth surface and decreased defects. The present invention provides a process for manufacturing a magnetic hard disc substrate made of an aluminum alloy which satisfies the above requirements and also provides a disc substrate which has a surface roughness Ra≦0.5 nm and is free from abrasive flaws with a depth of 5 nm or more and from micro-waviness or provides a disc substrate which has a surface roughness Ra≦5 angstroms and a surface roughness Rmax≦80 angstroms and in which the number of scratches with a depth of 50 angstroms or more is 5 or less and the number of pits with a depth of 50 angstroms or less is 5 or less in the surface.

BACKGROUND OF THE INVENTION

a) Field of the Invention

This invention relates to a Ni--P electroless-plated magnetic hard discsubstrate made of an aluminum alloy which is a high capacity-type, suchas those using a MR head or the like, and a process for manufacturingthe magnetic disc.

b) Description of the Prior Art

A magnetic hard disc substrate can be generally obtained by punching aroll coil made of an aluminum alloy to prepare a blank, grinding theblank to remove flaws, waviness, and the like to obtain a substrate,then electroless plating the substrate with Ni--P, finish-polishing theplated substrate, then forming a magnetic film by magnetic sputtering,and coating the magnetic film with an overcoat to obtain a sputteringmedia. In order to allow the surface roughness and the flatness to fallin prescribed ranges respectively in these steps, the grinding of themagnetic hard disc substrate (blank) prior to the Ni--P electrolessplating is performed by the following method: the substrate is put ontoa polishing board to which a nonwoven fabric polishing cloth made of anorganic polymer is applied and (a) the polishing operation is performedunder a fixed pressure while supplying, to the polishing face, apolishing solution produced by dispersing metal oxide particles, such asalumina, titania, and zirconia with an average grain size of 0.3 to 5 μmin an organic acid-type etchant wherein the polishing operation consistsof a first stage polishing using abrasive grains of a larger size andsuccessively second stage polishing using abrasive grains of a smallersize; or (b) the polishing operation is performed using a polishingsolution produced by dispersing colloid particles of silica, zirconia,titania, or the like, with an average grain size of 0.01 to 0.3 μm in anacid-type or alkali-type etchant.

In general, the magnetic hard disc substrate made of an aluminum alloywhich is electroless plated with Ni--P and is used for a hard disc ofmemory devices, such as a computer, is improving in recording densityfrom year to year. There is a great demand for more improvement in therecording density. There is also a tendency to a to higher density andlarger capacity. It is therefore important to finish the magnetic harddisc substrate used in these fields so that the substrate has theprescribed surface roughness and flatness. Particularly, the realizationof high density recording is due to a remarkable progress in lowfloatation technologies based on the improvement of the head, whichrequires the reduction of the interval between the head and the medium.With the reduction in the interval, the magnetic disc needs to have asmooth surface and mostly reduced surface defects. Specifically, thefollowing polishing qualities are required: the surface roughness Ra≦5angstroms, the surface roughness Rmax≦80 angstroms, and the surface isfree from scratches with a depth of 50 angstroms or more and from pitswith a depth of 50 angstroms or more. In the conventional method inwhich the polishing is performed using a polishing solution containingabrasive grains composed of a metal oxide such as alumina, titania, orzirconia, particles with a large grain size cannot be prevented fromgetting slightly mixed therein during a classification stage becausethese oxides are produced by grinding a massive raw material andclassifying the ground material. The contaminant particles with a largegrain size cause the production of scratches with a depth of 50angstroms.

In the method (a) among the aforementioned conventional methods, thoughthe waviness of the substrate decreases, the obtained value of Ra isabout 10 angstroms and polishing flaws with a depth as long as 100 μm ormore remains. In the method (b), in turn, the obtained value of Ra isabout 3 angstroms and the polishing flaws decrease in depth to about 50angstroms or less, but the waviness of the substrate remains. Also, thepolishing speed is low, which requires a polishing time as long as about5 minutes or more to attain the object Ra value. Magnetic hard discsubstrates obtained after the polishing step the conventional methodsinevitably have the features in that the surface roughness Ra is in arange from 7 to 15 angstroms, the surface roughness Rmax is in a rangefrom 80 to 150 angstroms, and several scratches with a depth of 80 to150 angstroms, some pits with a depth of 100 angstroms or less, andmicro-waviness are produced. When using a suspension of knownmicro-sized and uniform silica particles to polish in order to avoidthis situation, it takes a long time to polish because the particles aresmall. If erosive chemicals are added to accelerate the polishing, thesilica will gel to cause the silica particles to lose their uniformity.Thus, a surface with the surface roughness Ra≦5 angstroms and thesurface roughness Rmax≦80 angstroms cannot be achieved at present.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the above prior artproblems and to provide a magnetic hard disc substrate with a smoothsurface and decreased defects which makes it possible to achieve highdensity recording and also to provide a process for manufacturing themagnetic disc.

Another object of the present invention is to provide a magnetic harddisc substrate which has a surface roughness Ra≦5 angstroms and is freefrom scratches (grinding flaws) with a depth of 50 angstroms or more andfrom micro-waviness.

A further object of the present invention to provide a magnetic discwhich has a surface roughness Ra≦5 angstroms is a surface roughnessRmax≦80 angstroms and free from scratches with a depth of 50 angstromsor more and from pits with a depth of 50 angstroms or more.

A still further object of the present invention is to provide a processfor manufacturing the magnetic hard disc substrate in a short time andin a simple manner.

A still further object of the present invention relates to the aboveprocess using silica abrasive grains and is to provide a process formanufacturing the magnetic hard disc substrate having excellent surfaceproperties in an efficient manner by adding, to the polishing agent,additives for avoiding gelation and for improving the rate of polishing.

Still further objects will be clear from the descriptions herein below.

The present invention resides to a polishing process in succession to aNi--P electroless plating process in a process for manufacturing analuminum alloy magnetic hard disc substrate and comprises: dividing thepolishing process into plural polishing stages, polishing a subjectmaterial using a polishing solution containing metal oxide abrasivegrains with an average grain size of 0.3 to 5 μm in a polishing stagepreceding a final polishing stage, and polishing the treated subjectmaterial using a polishing solution containing colloid particles with agrain size of 0.01 μm to 0.3 μm in the final polishing stage. It isnoted that the polishing process using a metal oxide abrasive grains,which is followed by the final polishing process, may be divided into aplurality of polishing steps using particles with different grain sizesrespectively.

According to a further aspect of the present invention, there isprovided a process for manufacturing a magnetic hard disc substratecomprising: processing a subject material until the surface roughness Rais 15 angstroms or less and the surface roughness Rmax is 200 angstromsor less and then polishing the treated subject material using acolloidal abrasive agent containing silica particles with a grain sizeof 0.5 μm or less, to which are added additives for preventing gelationand accelerating the rate of polishing.

Preferably the above additive contains 0.01 mol/l of a trivalent ironion as an inorganic acid salt or organic acid salt in the presentinvention.

Other examples of the additive include 0.1 to 2.0 mol/l of hydroxyaceticacid, 0.1 to 2.0 mol/l of molybdenum oxide (hexavalent), 0.01% or moreof hydrogen peroxide, 0.03 to 4.0 mol/l of aluminum nitrate, 0.03 to 2.0mol/l of nitric acid, or a mixture thereof.

In the present invention having the above structure, a disc substratewhich has a surface roughness Ra≦5 angstroms and is almost free fromabrasive flaws with a depth of 50 angstroms or more and micro-wavinesscan be obtained.

In addition, a disc substrate having the polished qualities, in whichthe surface roughness Ra is 5 angstroms or less and the surfaceroughness Rmax is 80 angstroms or less and the number of scratches witha depth 50 angstroms or more and pits with a depth of 50 angstroms orless are both reduced to 5 or less, can be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be explained in detail.

A first process of the present invention comprises a polishing processusing abrasive grains composed of a metal oxide, the polishing processbeing carried out prior to the final polishing process. In the polishingprocess prior to the final polishing process, a subject substrate ispolished using a polishing solution containing abrasive grains of ametal oxide such as alumina, titania, zirconia, or the like, theabrasive grains having a grain size of 0.3 to 5 μm. When the averagegrain size of the metal oxide abrasive grains is less than 0.3 μm, thepolishing rate is low and hence it takes a long polishing time to reachthe intended polished finish, though the surface roughness of thesubject substrate is improved. On the other hand, when the average grainsize exceeds 5 μm, the polishing rate is high but the surface is moreroughened and abrasive flaws increases. Incidentally, this polishingprocess may be carried out in a plurality of polishing stages.Illustrating, for example, a case of 2 stages, the process includes afirst stage of polishing using metal oxide abrasive grains with anaverage grain size of 0.5 to 5 μm and a second stage of polishing usingmetal oxide abrasive grains with an average grain size of 0.3 to 1.5 μm.In this case, the second polishing stage is for a treating processpreceding the final polishing process.

The substrate which has been roughly finished to the given precision andtreated so as to be free from waviness and plating flaws is thensubjected to the final polishing process. This final polishing processis performed using, as the polishing material, a polishing solutioncontaining colloid particles of silica, titania, zirconia, or the likewith an average grain size of 0.01 to 0.3 μm to restrain the productionof abrasive flaws to the extreme. If the average grain size of thecolloid particles of silica, titania, zirconia, or the like is smallerthan 0.01 μm, the polishing process requires a long polishing time sothat it is unpractical. On the other hand, if the average grain size ofthe colloid particles exceeds 0.3 μm, the surface roughness Ra of themagnetic hard disc substrate is more than 50 angstroms.

The metal oxide abrasive grains used in the polishing process precedingthe final polishing process is dispersed in an aqueous solution in aproportion of 1 to 40%. The aqueous solution is preferably adjusted toan acidic range of a pH of 2 to 6 using nitric acid, phosphoric acid,sulfamic acid, or the like. The colloid particles used in the finalpolishing process is dispersed in an aqueous solution in a proportion of1 to 40% similarly. The aqueous solution is preferably adjusted to anacidic range of a pH of 2 to 6 using nitric acid, phosphoric acid,sulfamic acid, or the like or to an alkaline range using sodiumhydroxide.

The abrasive grains used for the polishing in each stage graduallydecrease in size with an increase in the number of stages. The polishingin each stage may be continuously performed using the same polishingmachine and abrasive grains with different sizes. It is howeverdesirable that each different polishing machine be used every polishingstage and the substrate be washed every time when the polishing in eachstage is completed to prevent the contamination of the abrasive grainsused in the antecedent stage. These devised measures make it possible toprepare a magnetic hard disc substrate which has a surface roughnessRa≦5 angstroms and is free from abrasive flaws with a depth of 50angstroms or more and from micro-waviness.

Another embodiment of the method of the present invention comprises astep of processing the substrate using an abrasive agent of alumina, forexample, with an average grain size of 0.3 to 2.0 μm according to aknown method until the surface roughnesses Ra and Rmax become 15angstroms or less and 200 angstroms or less and, preferably, 10angstroms or less and 100 angstroms or less, respectively, and a step ofpolishing using a colloidal abrasive agent containing an additivepossessing gelation preventive and polishing promoting activities andsilica particles with a grain size of 0.5 μm or less. Specifically,because it takes a long polishing time to obtain a substrate with theprescribed roughness only by using a colloidal abrasive agent containingsilica particles, first the substrate is processed using an abrasiveagent of alumina or the like, for example, with a grain size of 0.3 to2.0 μm, until the surface roughnesses Ra and Rmax become 15 angstroms orless and 200 angstroms or less and, preferably, 10 angstroms or less and100 angstroms or less respectively. After that, the substrate ispolished using a colloidal abrasive agent containing silica particles.The silica particles used in the present invention are preferably asilica sol prepared by a known preparative method in which the silicasol is produced from an aqueous silicic acid solution. Specifically, thefinal polishing process is performed using spherical or spheroidalsilica abrasive grains with a uniform grain size of 0.5 μm or less, andpreferably, 0.05 to 0.3 μm, which cannot be attained by pulverization orclassification. A prescribed amount of additives is added to theabrasive agent used in the polishing process using silica for thepurpose of preventing gelation and outstandingly improving the polishingrate, which is a significant feature of the present invention.

The aforementioned additive used in the present invention is any one ofthe following materials:

(a) 0.01 mol/l or more and preferably 0.01 to 3 mol/l of a salt of atrivalent iron and an inorganic acid or organic acid; wherein examplesof an inorganic acid include hydrochloric acid, sulfuric acid, nitricacid, and phosphoric acid; examples of the organic acid include citricacid, acetic acid, and oxalic acid; the amount of each of theseinorganic acids or organic acids is controlled in such a manner that itis sufficient to form a salt but not excessive to avoid the existence offree ions of the acid. If a salt containing less than 0.01 mol/l oftrivalent iron ions is added, the effect of iron ions is insufficient,whereas if the amount of trivalent ions exceeds 3 mol/l, the addition ofthe iron ions rather imparts an adverse effect.

(b) 0.1 to 2.0 mol/l of hydroxyacetic acid; if the amount ofhydroxyacetic acid is less than 0.1 mol/l, no effect of the addition canbe produced whereas an amount exceeding 2.0 mol/l rather imparts anadverse effect.

(c) 0.1 to 2.0 mol/l of molybdenum oxide (hexavalent); preferably 0.1 to1 mol/l of molybdenum oxide (hexavalent) is added; and if the amount ofmolybdenum oxide is less than 0.1 mol/l, only an insufficient effect ofthe addition can be produced whereas an amount exceeding 2.0 mol/lrather imparts an adverse effect.

(d) 0.01% or more of hydrogen peroxide, preferably 0.01 to 40%, and morepreferably, 0.01 to 30% of hydrogen peroxide, is added; and if theamount of hydrogen peroxide is less than 0.01%, only an insufficienteffect of the addition can be produced whereas an amount exceeding 40%rather imparts an adverse effect.

(e) 0.03 to 4.0 mol/l of aluminum nitrate, preferably 0.1 to 2.0 mol/lof aluminum nitrate is added; and if the amount of aluminum nitrate isless than 0.03 mol/l, only an insufficient effect of the addition can beproduced whereas an amount exceeding 4.0 mol/l rather imparts an adverseeffect.

(f) 0.03 to 2.0 mol/l of nitric acid, preferably 0.1 to 1.0 mol/l ofnitric acid is added; and if the amount of nitric acid is less than 0.03mol/l, only an insufficient effect of the addition can be producedwhereas an amount exceeding 2.0 mol/l rather imparts an adverse effect.

In this manner, according to the present invention, a disc substratehaving the polished qualities in which the surface roughness Ra is 5angstroms or less and the surface roughness Rmax is 80 angstroms or lessand the numbers of scratches with a depth 50 angstroms and pits with adepth of 50 angstroms are both reduced to 5 or less can be obtained.

Also, according to the present invention, a disc substrate having theabove polished qualities can be produced in a short time in a simplemanner. Thus, a magnetic hard disc substrate can be obtained which cancorrespond to a reduced interval between a head and a medium, thereduction of the interval resulting from the low floatation of the headwith an improvement in high density recording.

EXAMPLE

The present invention will be explained in more detail by way ofexamples, which are not intended to be limiting of the presentinvention.

Example 1

A donut-shaped blank (a product equivalent to 5086) made of an aluminumalloy with an outside diameter of 3.5 inches and a thickness of about 20μm was processed by lathe turning and subjected to Ni--P electrolessplating to form a plating coating with a thickness of about 20 μm,thereby preparing a sample. The sample was subjected to a polishingprocess according to the present invention. In Example 1, the sample waspolished for 3 minutes using a polishing solution produced by dispersing10 wt % of alumina with a maximum grain size of 1 μm and an averagegrain size of 0.8 μm in an aqueous solution adjusted to an acidic rangeof pH 4 using sulfamic acid. Next, the sample was polished for 1 minuteusing a polishing solution produced by dispersing 10 wt % of aluminawith a maximum grain size of 0.5 μm and an average grain size of 0.4 μmin an aqueous solution adjusted to an acidic range of pH 4 using thesame sulfamic acid as above. Then, the sample was finally polished for 3minutes using a polishing solution produced by dispersing 10 wt % ofsilica with a maximum grain size of 0.1 μm and an average grain size of0.05 μm in an aqueous 2% sodium hydroxide solution. In this polishing,25 substrates per one time were set in a polishing machine to carry outthe polishing and 5 samples among these 25 samples were optionallyselected and were designated as test samples.

Example 2

The same Ni--P electroless plated blank made of an aluminum alloy as inExample 1 was polished for 3 minutes using a polishing solution producedby dispersing 10 wt % of alumina with a maximum grain size of 1 μm andan average grain size of 0.8 μm in an aqueous solution adjusted to anacidic range of pH 4 using sulfamic acid. Then, the sample was finallypolished for 6 minutes using a polishing solution produced by dispersing10 wt % of silica with a maximum grain size of 0.1 μm and an averagegrain size of 0.05 μm in an aqueous 2% sodium hydroxide solution. Inthis polishing, 5 samples optionally selected and were designated astest samples in the same manner as in Example 1.

Comparative Examples 1 to 3

For comparison, the same Ni--P plated blanks made of an aluminum alloyas in Example 1 were polished in the following different conditions; for3 minutes using a polishing solution produced by dispersing 10 wt % ofalumina with a maximum grain size of 1 μm and an average grain size of0.8 μm in an aqueous solution adjusted to an acidic range of pH 4 usingsulfamic acid (Comparative Example 1); for 6 minutes using a polishingsolution produced by dispersing 10 wt % of alumina with a maximum grainsize of 0.5 μm and an average grain size of 0.4 μm in the same aqueoussolution as above (Comparative Example 2); and for 2 minutes using apolishing solution produced by dispersing 10 wt % of silica with amaximum grain size of 0.1 μm and an average grain size of 0.05 i m in anaqueous 2% sodium hydroxide solution (Comparative Example 3). In thispolishing, 5 samples were optionally selected and were designated astest samples in the same manner as in Example 1.

Measurements were made to determine the surface roughness (Ra), waviness(Wca), and abrasive flaws of the magnetic hard disc substrates obtainedin Examples 1 and 2 and Comparative Examples 1 to 3. Test conditions ofthese measurements are shown in the following. Each test was made for 5test samples of each example and an average of the measured results ofthese 5 test samples represents the result of each example.

* Surface roughness

Test machine: Tencor P12 manufactured by Tencor Co., Ltd.

Tracer diameter: 0.2 μm

Scan Length: 250 μm

Scan Speed: 5 μm/s

Cut off: 25 μm

* Waviness: determined according to a standard based on waviness (Wca)of the center line of a filtered wave.

Test machine: Tencor P12 manufactured by Tencor

Tracer diameter: 0.2 μm

Scan Length: 5 mm

Scan Speed: 400 μm/s

Cut off: 800 μm

"⊚", "◯", "Δ", and "×" in Table 1 indicate that Wca as a standard isless than 2 nm, from 2 to 5 nm, 5 to 10 nm, and more than 10 nm,respectively, wherein representations including and above "◯" show"pass".

* Depth of abrasive flaws:

Test machine: MHT-3 System manufactured by WYKO

Measurement magnification: ×400

PSI mode (Microregion measuring mode)

"⊚", "◯", "Δ", and "×"in Table 1 indicate that the depth is less than 3nm, from 3 to 5 nm, 5 to 10 nm, and more than 10 nm, respectively,wherein representations including and above "◯" show "pass".

                  TABLE 1                                                         ______________________________________                                               Polished                                                                             Surface                                                                amount roughness  Waviness Abrasive                                           (μm)                                                                              (Ra) (nm)  (Wcs)    flaw                                        ______________________________________                                        Ex. 1    1.2      0.3        ⊚                                                                     ⊚                          Ex. 2    1.2      0.4        ∘                                                                        ∘                             Comp. Ex. 1                                                                            1.0      1.3        ∘                                                                        x                                         Comp. Ex. 2                                                                            0.5      0.8        .increment.                                                                          x                                         Comp. Ex. 3                                                                            0.4      0.4        x      x                                         ______________________________________                                    

Example 3

A donut-shaped blank (a product equivalent to 5086) made of an aluminumalloy with an outside diameter of 3.5 inches and a thickness of about 20μm was processed by lathe turning and subjected to Ni--P electrolessplating to form a plating coating with a thickness of about 20 μm,thereby preparing a sample. The sample was polished according to thepresent invention. The surface roughness Ra of the Ni--Pelectroless-plated substrate was 80 angstroms. The substrate waspolished using an alumina abrasive agent with a grain size of 1.3 μmuntil the surface roughnesses Ra and Rmax became 22 angstroms and 170angstroms respectively. Then, using another machine, the substrate waspolished by an alumina abrasive agent with a grain size of 0.5 μm untilthe surface roughnesses Ra and Rmax became 10 angstroms and 100angstroms respectively. The resulting processed material was used as atest material which was then subjected to final polishing. Abrasivegrains used in the final polishing included, silica abrasive grains withgrain sizes of 0.1 μm, 0.05 μm, and 0.6 μm and, for comparison, analumina abrasive agent with a size of 0.5 μm. In the tests, the abrasivegrains used in the Examples contained iron chloride (trivalent), ironoxalate (trivalent), and iron citrate (trivalent) in varied amounts, asshown in Table 1, for the purposes of preventing gelation and improvingthe polishing rate. Each measured Ra, Rmax, scratches, pits, andpresence of projections of the substrate surface and polishing rate werecompared. The integrated evaluation of these comparisons are shown inTable 2.

In Table 2, in the evaluation of scratches, "×" indicates that thenumber of scratches is more than 5 per face and the depth is more than 8nm, "◯" indicates that the number of scratches is from 2 to 5 per faceand the depth is from 4 to 8 nm, and "⊚" indicates that the number ofscratches is less than 2 per face and the depth is less than 4 nm. Inthe evaluation of pits, "×", indicates that the number of pits is morethan 5 per face and the depth is more than 10 nm, "◯" indicates that thenumber of pits is from 1 to 5 per face and the depth is from 2 to 10 nm,and "⊚" indicates that no pit was observed. The projection means aconvex portion (on the surface) with 100 angstroms or more in heightdetermined from an average line of the roughness curve in AFM and with30 μm or less in length in a sectional direction, the convex portionconsisting of foreign materials adhering to the surface and having astrength sufficient to destroy the head when it collapses with the head.In the evaluation of the projection, "⊚" indicates that the number ofprojections is less than one for the surfaces and back faces of 10substrates, "◯" indicates that the number of projections is less thanone for the surfaces and back faces of 2 to 9 substrates, and "×"indicates the number of projections is more than one for the surface andback face of one substrate. For the evaluation of polishing rate, "×","◯", and "⊚" in Table 1 indicate that the rate is less than 0.01 μm/min,from 0.01 to 0.05 μm/min, and more than 0.05 μm/min, respectively.

Example 4

The same procedures and evaluations as in Example 3 were performedexcept that the additive was replaced with hydroxyacetic acid. In thisexample, the amount and the grain size of hydroxyacetic acid as theadditive were varied. These results are shown in Table 2.

Example 5

The same procedures and evaluations as in Example 3 were performedexcept that the additive was replaced with molybdenum oxide. In thisexample, the amount and the grain size of molybdenum oxide as theadditive were varied. These results are shown in Table 2.

Example 6

The same procedures and evaluations as in Example 3 were performedexcept that the additive was replaced with 10% or 30% hydrogen peroxide.These results are shown in Table 2.

Example 7

The same procedures and evaluations as in Example 3 were performedexcept that the additive was replaced with aluminum nitrate. In thisexample, the amount of aluminum nitrate as the additive was varied.These results are shown in Table 2.

Example 8

The same procedures and evaluations as in Example 3 were performedexcept that the additive was replaced with nitric acid. In this example,the amount and the grain size of nitric acid as the additive werevaried. These results are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                                                         Polishing                                                                          Integrated          *     Additive                  Ra (nm)                                                                           Rmax (nm)                                                                           Scratch                                                                           Pit                                                                             Projection                                                                         rate evaluation          __________________________________________________________________________    Ex. 3                                                                            In 0.5 mol/l iron chloride grain size: 0.1 μm                                                           ∘ (0.4)                                                               ∘ (4.0)                                                                 ⊚                                                                  ⊚                                                                ⊚                                                                   ∘                                                                      ⊚       In 0.5 mol/l iron chloride grain size: 0.05 μm                                                          ⊚ (0.3)                                                            ⊚ (3.4)                                                              ⊚                                                                  ⊚                                                                ⊚                                                                   ∘                                                                      ⊚       In 2.5 mol/l iron chloride grain size: 0.1 μm                                                           ∘ (0.4)                                                               ∘ (4.1)                                                                 ⊚                                                                  ⊚                                                                ⊚                                                                   ⊚                                                                   ⊚       In 0.5 mol/l iron oxalate grain size: 0.1 μm                                                            ∘ (0.4)                                                               ∘ (4.2)                                                                 ⊚                                                                  ⊚                                                                ∘                                                                      ∘                                                                      ∘          In 0.5 mol/l iron citrate grain size: 0.1 μm                                                            ∘ (0.4)                                                               ∘ (4.1)                                                                 ⊚                                                                  ⊚                                                                ⊚                                                                   ∘                                                                      ⊚       Out                                                                              0.5 mol/l iron chloride grain size: 0.6 μm                                                           x (1.2)                                                                           x (13.3)                                                                            ∘                                                                     ∘                                                                   ∘                                                                      ⊚                                                                   x                   Ex. 4                                                                            In 0.1 mol/l hydroxyacetic acid grain size: 0.1 μm                                                      ∘ (0.4)                                                               ∘ (4.1)                                                                 ⊚                                                                  ⊚                                                                ⊚                                                                   ∘                                                                      ∘          In 1.0 mol/l hydroxyacetic acid grain size: 0.1 μm                                                      ⊚ (0.4)                                                            ⊚ (3.5)                                                              ⊚                                                                  ⊚                                                                ⊚                                                                   ⊚                                                                   ⊚       out                                                                              3.0 mol/l hydroxyacetic acid grain size: 0.1 μm                                                      ∘ (0.5)                                                               ∘ (4.8)                                                                 ∘                                                                     x ∘                                                                      ⊚                                                                   x                      out                                                                              1.0 mol/l hydroxyacetic acid grain size: 0.6 μm                                                      x (0.9)                                                                           x (10.1)                                                                            x   ∘                                                                   ∘                                                                      ⊚                                                                   x                   Ex. 5                                                                            In 0.1 mol/l molybdenum oxide grain size: 0.1 μm                                                        ∘ (0.5)                                                               ∘ (4.4)                                                                 ⊚                                                                  ⊚                                                                ⊚                                                                   ∘                                                                      ⊚       In 1.0 mol/l molybdenum oxide grain size: 0.1 μm                                                        ⊚ (0.4)                                                            ⊚ (3.7)                                                              ⊚                                                                  ⊚                                                                ⊚                                                                   ⊚                                                                   ⊚       out                                                                              3.0 mol/l molybdenum oxide grain size: 0.1 μm                                                        ∘ (0.6)                                                               ∘ (5.8)                                                                 x   x ∘                                                                      ⊚                                                                   x                      out                                                                              1.0 mol/l molybdenum oxide grain size: 0.6 μm                                                        x (1.2)                                                                           x (15.7)                                                                            ∘                                                                     ∘                                                                   ∘                                                                      ⊚                                                                   x                   Ex. 6                                                                            In 30% hydrogen peroxide, grain size: 0.1 μm                                                            ∘ (0.4)                                                               ∘ (4.0)                                                                 ⊚                                                                  ⊚                                                                ⊚                                                                   ∘                                                                      ⊚       In 30% hydrogen peroxide, grain size: 0.05 μm                                                           ⊚ (0.3)                                                            ⊚ (3.4)                                                              ⊚                                                                  ⊚                                                                ⊚                                                                   ∘                                                                      ⊚       In 10% hydrogen peroxide, grain size: 0.1 μm                                                            ∘ (0.4)                                                               ∘ (4.1)                                                                 ⊚                                                                  ⊚                                                                ⊚                                                                   ∘                                                                      ∘          Out                                                                              30% hydrogen peroxide, alumina abrasive grain size: 0.5                                                 x (1.0)                                                                           x (10.3)                                                                            x   x x    ∘                                                                      x                      Out                                                                              30% hydrogen peroxide, grain size: 0.6 μm                                                            x (1.2)                                                                           x (13.3)                                                                            ∘                                                                     ∘                                                                   ∘                                                                      ⊚                                                                   x                   Ex. 7                                                                            In 0.1 mol/l aluminum nitrate grain size: 0.1 μm                                                        ∘ (0.4)                                                               ∘ (3.9)                                                                 ⊚                                                                  ⊚                                                                ⊚                                                                   ∘                                                                      ∘          In 1.5 mol/l aluminum nitrate grain size: 0.1 μm                                                        ⊚ (0.3)                                                            ⊚ (3.5)                                                              ⊚                                                                  ⊚                                                                ⊚                                                                   ∘                                                                      ⊚       Out                                                                              5.0 mol/l aluminum nitrate grain size: 0.1 μm                                                        ∘ (0.5)                                                               ∘ (5.4)                                                                 x   x x    ⊚                                                                   x                      Out                                                                              1.5 mol/l aluminum nitrate grain size: 0.6 μm                                                        x (1.0)                                                                           x (12.9)                                                                            ∘                                                                     ∘                                                                   ∘                                                                      ⊚                                                                   x                   Ex. 8                                                                            In 0.03 mol/l nitric acid grain size: 0.1 μm                                                            ∘ (0.4)                                                               ∘ (4.0)                                                                 ⊚                                                                  ⊚                                                                ⊚                                                                   ∘                                                                      ∘          In 0.5 mol/l nitric acid grain size: 0.1 μm                                                             ⊚ (0.3)                                                            ⊚ (3.4)                                                              ⊚                                                                  ⊚                                                                ⊚                                                                   ∘                                                                      ⊚       Out                                                                              2.5 mol/l nitric acid grain size: 0.1 μm                                                             ∘ (0.5)                                                               ∘ (5.7)                                                                 ⊚                                                                  ⊚                                                                x    ⊚                                                                   x                      Out                                                                              0.5 mol/l nitric acid grain size: 0.6 μm                                                             x (1.1)                                                                           x (12.4)                                                                            ∘                                                                     ∘                                                                   ∘                                                                      ⊚                                                                   x                      Out                                                                              0.5 mol/l hydrochloric acid grain size: 0.1 μm                                                       ∘ (0.4)                                                               ∘ (4.2)                                                                 ⊚                                                                  x x    ∘                                                                      x                   Comp. Ex. (Alumina, 0.5 μm)  x (1.0)                                                                           x (10.3)                                                                            x   x x    ∘                                                                      x                   Non additive, grain size: 0.1 μm                                                                           ∘ (0.5)                                                               ∘ (4.7)                                                                 ∘                                                                     ∘                                                                   ∘                                                                      x    x                   __________________________________________________________________________     Integrated evaluation (⊚: Excellent, ∘:            Practical, x: Unpractical)                                                    *: In: described in examples, Out: not described in examples             

What is claimed is:
 1. A process for manufacturing a magnetic hard discsubstrate comprising the steps of:plating a Ni--P coating on asubstrate; polishing the Ni--P plated substrate with a polishingsolution containing metal oxide abrasive grains to form a surface onsaid plated substrate having a surface roughness R_(a) of no more than15 angstroms and a surface roughness R_(max) of no more than 200angstroms; and then polishing the Ni--P plated substrate with acolloidal abrasive agent containing at least 0.01% of hydrogen peroxideand silica particles having a grain size not larger than 0.5 μm, saidhydrogen peroxide preventing gelation and accelerating the rate ofpolishing.
 2. The process according to claim 1, wherein the surfaceroughness R_(a) is no more than 10 angstroms and the surface roughnessR_(max) is no more than 100 angstroms.
 3. The process according to claim1, wherein said silica particles comprise spherical or spheroidal silicagrains having a grain size of 0.05 to 0.3 μm.
 4. A process formanufacturing a magnetic hard disc substrate comprising the stepsof:plating a Ni--P coating on a substrate; polishing the Ni--P platedsubstrate with a polishing solution containing metal oxide abrasivegrains to form a surface on said plated substrate having a surfaceroughness R_(a) of no more than 15 angstroms and a surface roughnessR_(max) of no more than 200 angstroms; and then polishing the Ni--Pplated substrate with a colloidal abrasive agent containing at least0.01 mol/l of a trivalent iron ion in the form of an inorganic ororganic acid salt and silica particles having a grain size of no morethan 0.5 μm, said trivalent iron ion preventing gelation andaccelerating the rate of polishing.
 5. The process according to claim 4,wherein the content of the trivalent iron ion is from 0.01 to 3 mol/l.6. A process for manufacturing a magnetic hard disc substrate comprisingthe steps of:plating a Ni--P coating on a substrate; polishing the Ni--Pplated substrate with a polishing solution containing metal oxideabrasive grains to form a surface on said plated substrate having asurface roughness R_(a) of no more than 15 angstroms and a surfaceroughness R_(max) of no more than 200 angstroms; and then polishing theNi--P plated substrate with a colloidal abrasive agent containing 0.1 to2.0 mol/l of hydroxyacetic acid and silica particles having a grain sizeof no more than 0.5 μm, said hydroxyacetic acid preventing gelation andaccelerating the rate of polishing.
 7. A process for manufacturing amagnetic hard disc substrate comprising the steps of:plating a Ni--Pcoating on a substrate; polishing the Ni--P plated substrate with apolishing solution containing metal oxide abrasive grains to form asurface on said plated substrate having a surface roughness R_(a) of nomore than 15 angstroms and a surface roughness R_(max) of no more than200 angstroms; and then polishing the Ni--P plated substrate with acolloidal abrasive agent containing 0.1 to 2.0 mol/l of molybdenum (VI)oxide and silica particles having a grain size of no more than 0.5microns, said molybdenum (VI) oxide preventing gelation and acceleratingthe rate of plating.
 8. The process of claim 7, wherein the colloidalabrasive agent contains 0.1 to 1 mol/l of molybdenum (VI) oxide.
 9. Theprocess of claim 1, wherein the content of hydrogen peroxide is from0.01 to 40%.
 10. The process of claim 1, wherein the content of hydrogenperoxide is from 0.01 to 30%.